1. Field of the Invention
The present invention relates to an optical filter for use in improving a quality of picture by optically erasing a matrix-like pixel pattern such as a liquid-crystal display (LCD) device by diffusing pixels and an image display apparatus and a liquid-crystal display device using such an optical filter. More particularly, the present invention relates to an optical filter or the like capable of an optimum pixel diffusion design by a combination of a diffraction grating and a birefringence plate while suppressing the occurrence of moire.
2. Description of the Related Art
Image display apparatus using a display device such as a liquid-crystal display device comprising arrays of a plurality of pixels have to increase the number of pixels in order to increase a resolution. The image display apparatus, however, cannot increase the number of pixels and reduce a space between pixels excessively from a standpoint of yield in production.
For this reason, if this kind of image display apparatus displays a large picture, a matrix-like pixel pattern based on a space between pixels (e.g. LCD black stripe portion) becomes conspicuous and a displayed picture becomes difficult for a viewer to see. In this kind of color image display apparatus, color filters of three primary colors (R, G, B) are arranged at a predetermined repetitive cycle in response to pixels. If this kind of color image display apparatus displays a large picture, a cycle in which a same color filter is disposed, i.e. same color dot cycle becomes conspicuous, and hence a displayed picture becomes difficult to see for the viewer.
CCD (charge-coupled device) cameras use an optical filter based on a double refraction (birefringence), and a space between pixels can be made inconspicuous by an electrical processing done by the low-pass filter.
On the other hand, since the image display apparatus using the display device in which a plurality of pixels are arrayed has to fill the space between the pixels after a signal was converted into a light signal, in actual practice, it is impossible to make the matrix-like pixel pattern and the same color dot cycle by the electrical processing. Although there is known a method of blurring a focus of a lens on the viewer side, such method is not preferable because a signal also is blurred together with the focus of the lens.
Heretofore, there has been a method of improving a quality of picture by optically erasing the matrix-like pixel pattern and the same color dot cycle based on the diffusion of pixels. In this method, there is used an optical filter comprising a diffraction grating or a birefringence plate attached to the front of the display device such as the liquid-crystal display device wherein each pixel of the display device is diffused into a plurality of pixels thereby to cancel the matrix-like pixel pattern and the same color dot cycle out optically, thus resulting in a quality of picture being improved.
However, when the diffraction grating is used as the optical filter, if the position at which the diffraction grating is mounted on the display device is determined, the grating pitch of the diffraction grating cannot be selected freely, thereby making it difficult to decrease a moire. This disadvantage can be avoided based on a difference between the moire and the focusing position on the image surface. However, if a viewer watches a picture through a lens having a small magnification, for example, and which has a small focus depth or a picture is displayed by an image display apparatus such as a spectacles-type monitor of which the focus is fixed, when an image is not properly focused, it is unavoidable that a moire becomes conspicuous.
Since the diffraction grating diffuses each pixel into three points at the lowest, if the display device of the image display apparatus has a number of pixels and it is not desired to increase the number of diffusions, it is not preferable to use the diffraction grating. Moreover, when a shift amount (length between adjacent diffusion points) is small, the grating pitch of the diffraction grating is widened so that an image distortion (matrix like distortion) caused by the cycle interval becomes conspicuous.
When a birefringence plate is used as an optical filter, if a shift amount is large, the thickness of the birefringence plate increases so that the optical filter becomes expensive. In particular, a crystal composing the low-pass filter used in the above-mentioned CDD camera has a small refractive index anisotropy. Therefore, when a birefringence material making the birefringence plate is a crystal, the thickness of the birefringence plate increases excessively, which is not suitable in actual practice.
Furthermore, since the birefringence plate diffuses each pixel into two points, if it is intended to increase the number of diffusions, the number of the birefringence plates also increases. As a result, the thickness of the birefringence plate increases excessively, which makes the optical filter expensive.